Surface plates are simple and extremely versatile. They provide a broad, smooth, flat reference surface that can be used to inspect incoming, in-process or finished parts. When used in combination with various gages and accessories, such as height gages, gage blocks, angle plates and squares, they can check a wide range of parameters, including length, flatness, squareness, straightness, angle, feature location and runout.
Surface plates are available in a wide range of sizes, from about 12 × 12 inches to 6 × 12 feet, and can weigh as much as 10 tons. They are available in three grades, the flatness tolerances for each grade varying with the size of the plate: AA (laboratory grade), A (inspection grade) and B (toolroom grade). Many surface plates can be ordered with ledges and threaded inserts, both of which make it easier to clamp workpieces or accessories to the surface. Granite is the most common material used in their construction; it is harder and denser than steel, has very little internal stress and is less subject to dimensional change due to temperature variations.
Any of the checks mentioned above can be done using a height gage and indicator as the measuring instrument and a gage block stack as the zero reference. However, there is a gaging technique that anyone can adapt to improve the overall performance of the measurement. It’s one that will improve those measurements when gaging performance to 50 microinches or 1 micron is required.
The typical process for making a measurement on a surface plate is to place the part in one spot on the surface, the gage block stack in another and the height gage in a third. This relationship is shown in Figure A. The height gage is first moved to the gage block along path (a) then on to the part along path (b). Once on the part, the deviation between the gage block and the part is read on the indicator.
Now, think about what we are trying to accomplish and what we are actually doing in terms of getting to a performance level of 50 microinches or 1 micron. As noted, surface plates are available in many different grades and, after use in the shop, exist in many different conditions. In other words, there is always some level of variation in the surface plate to begin with, and there will always be some out-of-flatness or waviness condition. Typically, a surface plate might see a total flatness tolerance of 200 microinches or 5 microns over its full surface. Plus, since the surface plate is being used in the shop, there is always the consideration of dirt, chips and scratches influencing the
Thus, if the performance we want to achieve is close to the flatness expected in the surface plate, then measuring just anywhere on the plate would not leave much tolerance available for comparing the master to the part. Since we are trying to get the most out of the measurement, we need to reduce as many sources of errors as possible.
Therefore, to keep the influence of waviness or localized surface plate error to a minimum, the best practice is to keep the distance between the gage block stack and the part to a minimum. This reduces the distance the height gage needs to be repositioned.
As shown in Figure B, placing the gage blocks as close as possible to the part enables the height gage to be moved very little to access the part. Minimizing the path minimizes the area of the surface plate used and thus eliminates the amount of waviness error by which the gage could be influenced.
By using this minimalist technique, we can reduce the chance that waviness will affect our measurement and can help achieve the results we are seeking.